Heat-seal paper having air permeabillty

ABSTRACT

A laminate integrated by firmly jointing a heat-seal layer in web form made from a mixture of synthetic pulp having a branched configuration and synthetic short fiber to a substrate layer made from natural fiber. The synthetic pulp is made from a resin composition comprised of ethylene-α,β-unsaturated carboxylic acid copolymer and polyethylene. This laminate has a high degree of air permeability and exhibits a high heat-seal strength and good hot tack even at a temperature of 130° C. On account of said properties it is adequately used as heat-seal paper suited for filter use, in particular raw material paper for tea bags.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a heat-seal paper having airpermeability, and more particularly relates to a heat-seal papersuitable for the manufacture of tea bags or bags for packagingsterilized apparatus and tools, desiccators, etc.

BACKGROUND OF THE INVENTION

[0002] The heat-seal paper conventionally used as the raw material paperfor tea bags, etc. used to be made from some type of mixed pulp composedof natural fiber and synthetic fiber. Such paper often impairedproductivity of automatic packaging machine due to adhesion of moltensynthetic fibers contained in it to the hot plate when fed to theheat-sealing section of the packaging machine. Coming into predominantuse in the industry these days is such double layered heat-seal paperconstructed of a heat-seal layer principally composed of synthetic fiberand a substrate layer principally composed of natural fiber, the formerbeing laminated on the latter.

[0003] This double layered type heat-seal paper is required to possessair permeability, to have its heat-seal layer firmly jointed to thesubstrate layer, and to have its heat-seal layer retain goodheat-sealability, hot tack, etc.

[0004] Meanwhile, the synthetic fiber used for the conventionalheat-seal layer is polyethylene fiber, polypropylene fiber, polyesterfiber, or conjugate fiber thereof. Heat-seal paper made from said fibershas good air permeability, but is said to impart only too low interlayerbond strength between the heat-seal layer and the substrate layer. Whatis pointed out as the factor to bring about low interlayer bond strengthis that said fiber makes it difficult for developing intertwistsinvolving fibers in the substrate layer because it is in the rod shapeand, therefore, is destitute of ramification.

[0005] As the automatic packaging machines have come to acquire veryhigh operating speeds recently, the industry is demanding in particularhigher degrees of heat-sealability and hot tack at low sealingtemperatures. Said fibers conventionally put in use for the heat-seallayer have not necessarily had sufficient heat-sealability and hot tackat low sealing temperatures, although they have at high sealingtemperatures.

[0006] Japanese Patent Publication No. 1969-16801 (corresponding foreignpatent: German Patent No. 1546330: British Patent No. 1091073) disclosesa fiber made from vinyl acetate-vinyl chloride copolymer (trade name“Vinyon”). There appears a description therein that said fiber can beused as a raw material for heat-seal paper to be used in the manufactureof tea bags. Although said heat-seal paper has good air permeability,heat-sealability, etc., it is faced with demands for further improvedproduct qualities so that it would not give rise to generation ofdioxines and other harmful substances when incinerated as wastes.

DESCRIPTION OF THE INVENTION

[0007] Accordingly it is an object of the present invention to provide aheat-seal paper which has a heal seal layer and a substrate layer firmlyjointed to each other and retains good air permeability.

[0008] It is the second object of the present invention to provide aheat-seal paper which imparts stable and good heat-sealability and hottack over a wide range of heat-sealing temperature range extending fromlow temperatures to high temperatures.

[0009] It is the third object of the present invention to provide aheat-seal paper which is few fear of generation of dioxines and otherharmful substances when incinerated as wastes, and thus contributes toprotection of the environments.

[0010] At large, the present invention relates to a heat-seal paperwhose technical details are as enumerated in items 1 to 24 as follows:

[0011] 1. A heat-seal paper having air permeability, which comprises alaminated body integrating a heat-seal layer (A) principally comprisedof a synthetic fiber having a branched configuration and a substratelayer (B) principally comprised of a natural fiber.

[0012] 2. A heat-seal paper having air permeability according to item 1,wherein said synthetic fiber having the branched configuration is apolyolefin synthetic pulp.

[0013] 3. A heat-seal paper having air permeability according to item 2,wherein said polyolefin synthetic pulp has an average fiber length of0.1 to 10 mm and a freeness of 700 cc or less.

[0014] 4. A heat-seal paper having air permeability according to item 2or 3, wherein said polyolefin synthetic pulp is constructed of a resincomposition comprised of 50 to 100% by weight of anethylene-α,β-unsaturated carboxylic acid copolymer containing 1 to 20%by weight of an α,β-unsaturated carboxylic acid and 0 to 50% by weightof a polyethylene resin.

[0015] 5. A heat-seal paper having air permeability according to eitherone of items 1 to 4, wherein said heat-seal layer (A) is comprised of 50to 100% by weight of the polyolefin synthetic pulp and 0 to 50% byweight of at least one type of fiber selected from the group consistingof polyethylene fiber, polypropylene fiber, polyester fiber andconjugate fiber thereof.

[0016] 6. A heat-seal paper having air permeability according to eitherone of items 1 to 4, wherein said heat-seal layer (A) is comprised of 50to 90% by weight of the polyolefin synthetic pulp and 10 to 50% byweight of at least one type selected from the group consisting ofpolyethylene fiber, polypropylene fiber, polyester fiber and conjugatefiber thereof.

[0017] 7. A heat-seal paper having air permeability, which is alaminated body integrating a heat-seal layer (A) and a substrate layer(B) principally containing natural fiber wherein said heat-seal layer(A) is comprised of 1 to 99% by weight of a polyolefin synthetic pulphaving a branched configuration and an average fiber length of 0.1 to 10mm and 1 to 99% by weight of a synthetic fiber having a fineness of 0.1to 10 deniers and an average fiber length of 0.1 to 30 mm, and saidsynthetic pulp is constructed of a resin composition comprised of 50to99% by weight of an ethylene-α,β-unsaturated carboxylic acid copolymercontaining 1 to 20% by weight of an α,β-unsaturated carboxylic acid and1 to 50% by weight of a polyethylene resin.

[0018] 8. A heat-seal paper having air permeability according to item 7,wherein said heat-seal layer (A) is comprised of 50 to 99% by weight ofthe polyolefin synthetic pulp and 1 to 50% by weight of the syntheticfiber.

[0019] 9. A heat-seal paper having air permeability according to item 7,wherein said heat-seal layer (A) is comprised of 50 to 90% by weight ofthe polyolefin synthetic pulp and 10 to 50% by weight of the syntheticfiber.

[0020] 10. A heat-seal paper having air permeability according to eitherone of items 7 to 9, wherein said synthetic fiber is at least one typeof fiber selected from the group consisting of polyethylene fiber,polypropylene fiber, polyester fiber and conjugate fiber thereof.

[0021] 11. A heat-seal paper having air permeability according to eitherone of items 7 to 10, wherein said ethylene-α,β-unsaturated carboxylicacid copolymer is an ethylene-acrylic acid copolymer or anethylene-methacrylic acid copolymer.

[0022] 12. A heat-seal paper having air permeability according to eitherone of items 7 to 11, wherein said polyethylene resin has a density of0.941 to 0.970.(g/cm³) and a melt flow rate of 0.1 to 100 (g/10 min.).

[0023] 13. A heat-seal paper having air permeability according to eitherone of items 7 to 12, wherein said polyolefin synthetic pulp has afreeness of 700 cc or less.

[0024] 14. A heat-seal paper having air permeability according to eitherone of items 7 to 13, wherein said heat-seal layer (A) has a thicknesscorresponding to a grammage of 1 to 10 (g/m²).

[0025] 15. A heat-seal paper having air permeability according to eitherone of items 7 to 14, wherein said substrate layer (B) has a thicknesscorresponding to a grammage of 10 to 30 (g/m²).

[0026] 16. A heat-seal paper having air permeability according to eitherone of items 7 to 15, wherein said substrate layer (B) is composed of anabaca pulp.

[0027] 17. A heat-seal paper having air permeability according to eitherone of items 7 to 16, wherein said laminated body is comprised of 15 to30% by weight of the heat-seal layer (A) and 70 to 85% by weight of thesubstrate layer (B).

[0028] 18. A heat-seal paper having air permeability according to eitherone of items 7 to 17, wherein said laminated body has a thicknesscorresponding to a grammage of 11 to 40 (g/m²).

[0029] 19. A heat-seal paper having air permeability according to eitherone of items 7 to 18, wherein said laminated body has a Frazier airpermeability of 100 to 300 (cm³/cm²/s).

[0030] 20. A heat-seal paper having air permeability according to eitherone of items 7 to 19, wherein said laminated body has a heat-sealstrength at a temperature of 130° C. of 100 to 300 (g/15 mm).

[0031] 21. A heat-seal paper having air permeability according to eitherone of items 7 to 20, wherein said laminated body has a hot tack asrepresented by a peel distance at a temperature of 130° C. is 1 to 100mm.

[0032] 22. A heat-seal paper having air permeability according to eitherone of items 7 to 21, wherein said laminated body is a filter bag paper.

[0033] 23. A heat-seal paper having air permeability according to eitherone of items 7 to 21, wherein said laminated body is a tea bag paper.

[0034] 24. A heat-seal paper having air permeability according to eitherone of items 7 to 23, wherein the heat-seal layer (A) and the substratelayer (B) constituting said laminated body do not substantially containany halogen atom.

[0035] In other words, the present invention relates to a heat-sealpaper having air permeability, which comprises a laminated bodyintegrating a heat-seal layer (A) principally comprised of a syntheticfiber having a branched configuration and a substrate layer (B)principally comprised of a natural fiber.

[0036] It is preferable that said synthetic fiber having the branchedconfiguration is a polyolefin synthetic pulp. It is particularlypreferable that it is constructed of a resin composition comprised of 50to 100% by weight of an ethylene-α,β-unsaturated carboxylic acidcopolymer containing 1 to 20% by weight of an α,β-unsaturated carboxylicacid and 0 to 50% by weight of a polyethylene resin.

[0037] It is also preferable that said heat-seal layer (A) is composedof 50 to 100% by weight, and preferably 50 to 90% by weight of thepolyolefin synthetic pulp and 0 to 50% by weight, and preferably 10 to50% by weight of at least one type of fiber selected from the groupconsisting of polyethylene fiber, polypropylene fiber, polyester fiberand conjugate fiber thereof.

[0038] The present invention relates to a heat-seal paper having airpermeability, which is a laminated body integrating a heat-seal layer(A) and a substrate layer (B) principally containing a natural fiber,wherein said heat-seal layer (A) is comprised of 1 to 99% by weight of apolyolefin synthetic pulp having a branched configuration and an averagefiber length of 0.1 to 10 mm and 1 to 99% by weight of a synthetic fiberhaving a fineness of 0.1 to 10 deniers and an average fiber length of0.1 to 30 mm, and said synthetic pulp is comprised of 50 to 99% byweight of an ethylene-α,β-unsaturated carboxylic acid copolymercontaining 1 to 20% by weight of an α,β-unsaturated carboxylic acid and1 to 50% by weight of a polyethylene resin.

[0039] It is furthermore preferable that said heat-seal layer (A) iscomprised of 50 to 99% by weight, and more preferably 50 to 90% byweight of the polyolefin synthetic pulp and 1 to 50% by weight, and morepreferably 10 to 50% by weight of the synthetic fiber.

[0040] For said heat-seal paper, it is preferable that said syntheticfiber is at least one type of fiber selected from the group consistingof polyethylene fiber, polypropylene fiber, polyester fiber andconjugate fiber thereof.

[0041] It is preferable that the ethylene-α,β-unsaturated carboxylicacid copolymer constituting polyolefin synthetic pulp is anethylene-acrylic acid copolymer or an ethylene-methacrylic acidcopolymer, wherein the polyethylene resin is a high density polyethylenehaving a density of 0.941 to 0.970 (g/cm³) and a melt flow rate of 0.1to 100 (g/10 min.).

[0042] The weight ratio of each layer constituting the laminated body ispreferably 15 to 30% by weight for the heat-seal layer (A) and 70 to 85%by weight for the substrate layer.

[0043] It is preferable that the breakdown by thickness of saidlaminated body is adjusted to 1 to 10 (g/m²) for the heat-seal layer(A), 10 to 30 (g/m²) for the substrate layer (B) and 11 to 40 (g/m²) asa whole, respectively, in terms of grammage.

[0044] The laminated body having said structure is suitable for the useas a heat-seal paper insofar as its Frazier air permeability is 100 to300 (cm³/cm²/s), its heat-seal strength is 100 to 300 (g/15 mm) at atemperature of 130° C., and its hot tack at a temperature of 130° C. asexpressed by the peel distance is 1 to 100 mm.

[0045] Since the heat-seal paper of the present invention exhibitsstable and good heat sealability and hot tack over a wide temperaturerange extending from low temperatures to high temperatures,demonstrating high effects particularly at low temperatures, and at thesame time possesses good air permeability and strong interlayer bondstrength between the heat-seal layer and the substrate layer, it may beutilized as filter bag paper, in particular as the raw material paperfor tea bags.

BRIEF DESCRIPTION OF DRAWINGS

[0046]FIG. 1 is a micrograph depicting a configuration of the syntheticpulp used in the present invention.

[0047]FIG. 2 is a micrograph depicting a configuration of the syntheticfiber used in the present invention.

[0048]FIG. 3 is a micrograph depicting a configuration of the heat-seallayer pertinent to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] A heat-seal paper having air permeability of the presentinvention is a laminated body which is comprised of basically aheat-seal layer and a substrate layer, each of which will be explainedin detail as follows:

Heat-seal Layer (A)

[0050] A heat-seal layer (A) constitutes a unit sheet contained of ablended fibrous layer comprising a polyolefin synthetic pulp and asynthetic fiber which as a whole does not readily get separated due tothe presence of branches in the synthetic pulp that are intertwistedwith the synthetic fibers.

[0051] (1) Polyolefin Synthetic Pulp

[0052] The polyolefin synthetic pulp is a pulp-like material formed frompolyolefin resin into short fiber having innumerous branches which hasan average fiber length of 0.1 to 10 mm, and preferably 0.5 to 5 mm.Innumerous branches are located on the surfaces of such fibers. FIG. 1is a micrograph depicting a configuration of the synthetic pulp. Themicrograph shows that fine short fibers get ramified from the surface ofa thick short fiber as is normally the case with a natural pulp.

[0053] While such synthetic pulp can be manufactured from various typesof polyolefin resin or a resin composition, it is preferable that it beformed from an ethylene-α,β-unsaturated carboxylic acid copolymer(a) inconsideration of such conditions that it may be readily formed into apulp-like material and possesses good heat-sealability and hot tack, andit is particularly preferably that it be formed from a resin compositioncomprised of (a) and polyethylene resin (b).

[0054] (2) Ethylene-α,β-Unsaturated Carboxylic Acid Copolymer

[0055] The Ethylene-α,β-unsaturated carboxylic acid copolymer is acopolymer derived by copolymerizing ethylene and an α,β-unsaturatedcarboxylic acid wherein α,β-unsaturated carboxylic acid unit iscontained by 1 to 20% by weight, preferably 3 to 15% by weight, and morepreferably 5 to 10% by weight. Insofar as the α,β-unsaturated carboxylicacid content is within said ranges, the synthetic pulp containing saidcopolymer imparts high levels of heat-sealability and hot tack over awide temperature range extending from low temperatures to hightemperatures and possesses sufficient freeness required of a wet processpaper.

[0056] As examples of the α,β-unsaturated carboxylic acid, the followingcompounds can be cited. Namely, acrylic acid, methacrylic acid, maleicacid, maleic anhydride, fumaric acid, itaconic acid, crotonic acid,himic anhydride, etc. Included in the category of α,β-unsaturatedcarboxylic acid for the present invention are the following. Namely,α,β-unsaturated carboxylic acid esters, which are derivatives ofα,β-unsaturated carboxylic acid, i.e., methyl acrylate, methylmethacrylate, monomethyl maleate, etc. Particularly preferred among themis acrylic acid or methacrylic acid, which is a monovalent acid.

[0057] Said ethylene-α,β-unsaturated carboxylic acid copolymer may be arandom copolymer of ethylene and the α,β-unsaturated carboxylic acid, ora graft copolymer derived by graft copolymerizing the α,β-unsaturatedcarboxylic acid to polyethylene. Particular preferred among them is therandom copolymer manufactured by directly copoly-merizing ethylene withthe α,β-unsaturated carboxylic acid. This type is conventionallymanufactured according to a high-pressure radical polymerizationprocess. In executing such copolymerization, the α,β-unsaturatedcarboxylic acids exemplified above may be used alone or in a combinationof two or more types.

[0058] The following copolymers can be cited as examples of theethylene-α,β-unsaturated carboxylic acid copolymer. Namely,ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer,ethylene-methacrylic acid-methyl methacrylate terpolymer, andethylene-acrylic acid-methyl methacrylate terpolymer.

[0059] The ethylene-α,β-unsaturated carboxylic acid copolymer usable forthe present invention may be so-called ionomer resin which is derived bypartially neutralizing at least part of the carboxylic groups containedin the ethylene-α,β-unsaturated carboxylic acid copolymer with suchmetal ion as Na⁺, K⁺, Zn⁺⁺, Ca⁺⁺, Mg⁺⁺, etc.

[0060] (3) Polyethylene Resin

[0061] Polyethylene resin may be an ethylene homopolymer or anethylene-60 -olefin copolymer. It is usable over a wide range extendingfrom a low density grade to a high density grade.

[0062] It is preferable that the ethylene homopolymer is in the rangesof 0.900 to 0.970 (g/cm³) for a density, and preferably 0.920 to 0.970(g/cm³); 0.1 to 100 (g/10 min.), and preferably 1 to 20 (g/10 min.) fora melt flow rate (MFR) as determined at a temperature of 190° C. andunder a load of 2.16 kg according to ASTM D-1238. Insofar as theethylene homopolymer having the density and the MFR falling within saidranges is used, a synthetic pulp of highly branched andwell-intertwisted pulp fibers may be obtained. Among different grades ofpolyethylene, high density polyethylene having the density of 0.941 to0.970 (g/cm³) is particularly preferred.

[0063] In the case of the ethylene-α-olefin copolymer, what is preferredis a crystalline polymer falling in the ranges of 90 to 99 mol. %, andpreferably 95 to 99 mol. % for ethylene content and 1 to 10 mol. %, andpreferably 1 to 5 mol. % for α-olefin content. Preferred for suchα-olefin are those olefins having 3 to 20 carbon atoms. Some examplesthereof are propylene, 1-butene, 1-pentene, 3-methyl-1-butene,4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and1-tetradecene.

[0064] It is preferable that such ethylene-α-olefin copolymer has adensity of 0.900 to 0.940 (g/cm³), and preferably 0.920 to 0.940 (g/cm³)and a melt flow rate (MFR) as determined at a temperature of 190° C. andunder a load of 2.16 kg according to ASTM D-1238 of 0.1 to 100 (g/10min.), and preferably 0.5 to 50 (g/10 min.). Insofar as theethylene-α-olefin copolymer to be used has the density and the MFRfalling in said ranges, synthetic pulp obtained from it exhibits highlyramified pulp fiber and favorable intertwists.

[0065] Particularly preferred among such ethylene-α-olefin copolymersare ethylene-1-butene copolymer, ethylene-4-methyl-1-pentene copolymerand ethylene-1-hexene copolymer having the density of 0.920 to 0.940(g/cm³) and the MFR of 1 to 10 (g/10 min.).

[0066] These ethylene homopolymer and ethylene-α-olefin copolymers canbe manufactured by polymerizing ethylene, and insofar as necessaryincluding α-olefin monomer in the reaction system in the presence ofZiegler-Natta catalyst or a metallocene catalyst according to suchconventional polymerization process as the gas phase process, the bulkprocess, the slurry process, and the solution process.

[0067] (4) Manufacture of Synthetic Pulp

[0068] The synthetic pulp usable for the present invention is formedinto pulp-like material from a resin composition preferably comprisingthe ethylene-α,β-unsaturated carboxylic acid copolymer (a) and thepolyethylene resin (b). The blending ratio (%) of the raw materials (a)and (b) are 50 to 100% by weight, preferably 50 to 99% by weight, andmore preferably 50 to 90% by weight, and furthermore preferably 60 to80% by weight for the raw material (a) and 0 to 50% by weight,preferably 1 to 50% by weight, more preferably 10 to 50% by weight, andfurthermore preferably 20 to 40% by weight for the raw material (b),wherein the sum of the raw materials (a) and (b) is to be 100% byweight.

[0069] Insofar as the blending ratio (%) of the raw materials (a) and(b) falls within said ranges, such synthetic pulp may be obtained thatwill give excellent freeness when it is made into paper according to thewet process, and also such synthetic pulp may be obtained as will impartstable and good heat-sealability and hot tack over a wide sealingtemperature range extending from low temperatures to high temperatures.

[0070] For manufacturing synthetic pulp from said raw material resincomposition, conventional processes can be employed. Such processes areexplained in detail in Encyclopedia of Chemical Technology, 3^(rd)ed.,Vol. 19, p.p. 420 to 425. For instance, there is introduced a processwhich comprises cutting melt-spun fiber into short elements, and thenbeating the resulting material; and another process which comprisesbeating materials obtained by melt-flash spinning or emulsion-flashspinning. In the pulp preparation process, various additives, such asantioxidants, antistatic agents, UV stabilizers, and pigments may beadded insofar as addition of such additives does not go counter to theobject of the present invention.

[0071] It is preferable that the synthetic pulp used in the presentinvention has an average fiber length of 0.1 to 10 mm, and preferably0.5 to 5 mm, and has a freeness conforming to Canadian Standard Freeness(CSF) of 700 cc or less. Preferred as the process for manufacturing suchpreferable synthetic pulp is the process of flash spinning a resinsolution or an emulsion. Particularly preferred is the emulsion-flashspinning process in which a polyvinyl alcohol (PVA) is utilized as thehydrophilic agent, since this process gives a pulp having such fiberconfiguration that promotes formation of intertwists among syntheticfibers in the heat-seal layer and natural fibers in the substrate layer.The preferable amount of the PVA to be added is 0.01 to 10% by weight tothe total quantity of the synthetic pulp including the PVA.

[0072] (5) Formation of the Heat-seal Layer (A)

[0073] The heat-seal layer (A) is a fibrous layer comprised of thepolyolefin synthetic pulp and the synthetic fiber.

[0074] The synthetic fiber for this use has a weight fineness of 0.1 to10 deniers, and preferably 0.5 to 8 deniers and an average fiber lengthof 0.1 to 30 mm, and preferably 0.5 to 25 mm, which is conventionallycategorized as short fiber. Such synthetic fiber may have a roundcross-section or oblong cross-section, and furthermore a polygonalcross-section. It may also have branches on its surface. FIG. 2 is amicrograph depicting an example of its configuration. It showsrod-shaped polypropylene short fiber having a round cross-section andsmooth surfaces.

[0075] The usable types of synthetic fiber are fibers having variousqualities such as polyolefin fiber, polyester fiber, polyamide fiber,etc. Particularly preferred among them are polyethylene fiber,polypropylene fiber, polyester fiber, or conjugate fibers thereof. Thestructure of the conjugate fiber may be of a sheath & core constructionor of a side-by-side construction. For example, the conjugate fibers ofthe sheath & core construction constructed of a polyethylene sheath anda polypropylene core is usable.

[0076] It is preferable that the blending ratio is 1 to 99% by weight,and preferably 50 to 99% by weight, and more preferably 50to 90% byweight, of the polyolefin synthetic pulp and 1 to 99% by weight,preferably 1 to 50% by weight, and more preferably 10 to 50% by weightof the synthetic fiber, so that the total quantity of the two becomes100% by weight. A blending ratio falling in said ranges is advantageousin that the heat-seal layer exhibits high heat-sealability and hot tackover a wide heat-sealing temperature range extending from lowtemperatures to high temperatures, and thus the bond to the substratelayer is made stronger.

[0077] As for the method of blending the synthetic pulp and thesynthetic fiber, blending can be accomplished without any difficulty bymechanically mixing the two during the paper making step. As specificmethods, conventional beating/refining machine such as a pulper and abeater can be utilized. A heat-seal layer having a sufficient heat-sealstrength and hot tack may be formed by adjusting the thickness of driedweb to a grammage, or basis weight (weight per unit area) of 1 to 20(g/m²), and preferably 1 to 10 (g/m²), and thus the obtained heat-seallayer exhibits a sufficient air permeability.

[0078]FIG. 3 is a micrograph depicting an example of the heat-seallayer. The heat-seal layer shown in the photograph is a fibrous layerformed by mechanically mixing 70% by weight of the synthetic pulp and30% by weight of a polypropylene short fiber. It may be clearly seenfrom this micrograph that synthetic pulp having innumerous branches onits surfaces intertwisted with rod-shaped polypropylene short fiberswhich have smooth surfaces.

Heat-seal Paper

[0079] (1) Structure of the Heat-seal Paper

[0080] A laminated body obtained by laminating the heat-seal layer (A)on a substrate layer (B) so as to integrate them into a united body maybe a double layered laminate of the (A)/(B) construction or a triplelayered laminate of the (A)/(B)/(A) construction. Such laminate can beutilized as a heat-seal paper. It is typically utilized extensively inthe form of the double layered laminate of the (A)/(B) construction.

[0081] The substrate layer (B) is a fibrous layer principally comprisedof a natural fiber and may additionally contain small amounts ofsynthetic fiber or synthetic pulp or the like. As the natural fiber forthis use, such natural fiber as is utilized for the conventional teabags or sterilized paper are usable. A few examples thereof are abacapulp, NBKP (needle bleached kraft pulp), and LBKP (leaf bleached kraftpulp). Particularly preferred among them is abaca pulp in view of itsair permeability and strength. The thickness of the substrate layervaries with the intended use of the heat-seal paper. Its suitablegrammage is in the range of 10 to 50 (g/m²), and preferably of 10 to 30(g/m²).

[0082] (2) Manufacture of the Heat-seal Paper

[0083] The heat-seal paper can be manufactured according to a processwhich comprises laminating the heat-seal layer (A) on at least one sideof the substrate layer (B) and thereupon firmly integrating the twolayers into a united body by heat treatment.

[0084] A detailed explanation is furnished as follows with reference toan example. The substrate layer and the heat-seal layer are individuallyformed using a sheet machine as separate wet webs and while the twolayers are in the wet state, the heat-seal layer is stacked onto atleast one side of the substrate layer, and the stack thus prepared isdried at a temperature of 40 to 110° C. using a heating device such as ahot oven or a hot roll, and thereupon the resulting sheet is subjectedto heat treatment at a temperature of 130 to 200° C. so as to be madeinto the heat-seal paper. In the course of said operation, the heat-seallayer (A) and the substrate layer (B) are bonded to each other firmly tohave a high interlayer bond strength as innumerous branches of syntheticpulp situated on the surface of the heat-seal layer are firmly bonded tothe fibers constituting the substrate layer.

[0085] The weight ratio of each layer is 15 to 30% by weight, andpreferably 15 to 25% by weight, of the heat-seal layer (A) and 70 to 85%by weight, and preferably 75 to 85% by weight of the substrate layer(B). And, the total thickness of the laminated body is so adjusted thatit will correspond to a grammage of 11 to 40 (g/m²).

[0086] (3) Properties of the Heat-seal Paper

[0087] It is preferable that an air permeability of the heat-seal paperhaving aforementioned structure is 100 to 300 (cm³/cm²/s), andpreferably 100 to 200 (cm³/cm²/s) in Frazier air permeability asdetermined using Frazier-model air permeability testing apparatus whichis prescribed in JIS L1004. The heat-seal paper having the airpermeability falling in said ranges can be favorably used in variousapplications for the filter use.

[0088] A heat-seal strength of said heat-seal paper at a temperature of130° C. falls in the range of 100 to 300 (g/15 mm), and preferably 120to 200 (g/15 mm). Insofar as the heat-seal strength of the heat-sealpaper is within said ranges, such heat-seal paper exhibits sufficientbond strength and hence it is applicable to extensive uses.

[0089] The heat-seal strength is determined according to the followingprocedure. Namely, two sheets of heat-seal paper are stacked in suchmanner that their respective heat-seal layers will touch face-to-faceeach other. Thereupon a 10 mm-wide seal bar controlled to a temperatureof 130° C. is pressed from the substrate layer side under a pressure of2 kg/cm² for 1 sec. After the heat-sealed area has been cooled to theroom temperature, a specimen of a 15 mm width cut out from it issubjected to such test that its heat-sealed area is peeled at acrosshead speed of 100 mm/min. so that its peel strength (g/15 mm) ismeasured. The measured value is recorded as the heat-seal strength.

[0090] A hot tack of this heat-seal paper at a temperature of 130° C. asexpressed in terms of a peel distance is 1 to 100 mm, and preferably 1to 70 mm. Insofar as the peel distance, which is an indicator of hottack, is within said ranges, speedy and stable packaging operation maybe continuously executed when this heat-seal paper is fed to anautomatic filling/packaging machine designed to accomplish both abag-making job using the heat-seal paper and a loading job by itself.

[0091] The peel distance adopted as the indicator of the hot tack wasdetermined according to the following procedure. Namely, two sheets ofheat-seal paper were stacked in such manner that their heat-seal layerswould touch face-to-face each other, and thereupon the stacked layerswere heat-sealed at a temperature of 130° C. for 0.5 sec. under apressure of 1 kg/cm², and immediately afterwards the peel distance ofthe heat-sealed area was measured under a load of 45 g.

[0092] As explained in the foregoing, the heat-seal paper of the presentinvention retains the high air permeability and the heat-seal layer andthe substrate layer are jointed to each other to have a high interlayerbond strength. When it is fed to a heat-seal section in an automaticpackaging machine, adhesion possibly caused in a molten state of theheat-seal paper to a hot plate in the machine may be prevented while itpossesses good hot tack. For said reason, the heat-seal paper of thepresent invention may enhance stable productivity of bags, and bagshaving high heat-seal strength may be manufactured on a continuousbasis.

EXAMPLES

[0093] Next, the present invention is further described with referenceto examples, but it should be construed that the invention is in no waylimited to those examples.

[0094] The following two types of resin were made available so as to beused in examples.

[0095] (1) Resin 1: Ethylene-methacrylic acid copolymer

[0096] MFR (at 190° C.) =1.5 (g/10 min.)

[0097] Melting point =99° C.

[0098] Methacrylic acid content=9% by weight

[0099] (2) Resin 2: High density polyethylene

[0100] MFR (at 190° C.)=13 (g/10 min.)

[0101] Melting point=135° C.

[0102] Density=0.965 (g/cm³)

[0103] Properties of the manufactured synthetic pulp and heat-seal paperwere measured according to the following test procedures.

[0104] (1) Average Fiber Length (Abb.: CFL)

[0105] The average fiber length per unit weight (mm) was determinedusing an automatic fiber length testing apparatus FS-200-modelmanufactured by Kajaani Co., Finland, and the measured value wasreported as the average fiber length in the unit of millimeter.

[0106] (2) Freeness (Abb.: CSF)

[0107] Canadian Standard Freeness was determined in accordance with JISP-8121, and the measured value was reported as the freeness value in theunit of cubic centimeter (cc).

[0108] (3) Air Permeability

[0109] Frazier air permeability was determined using a Frazier-model airpermeability testing apparatus as prescribed in JIS L1004, and themeasured value was reported as air permeability in the unit ofcm³/cm²/s.

[0110] (4) Heat-seal Strength

[0111] Two sheets of double layered hand made sheet were stacked in suchmanner that their heat-seal layers will touch face-to-face each otherand were heat-sealed for 1 sec under a pressure of 2 kg/cm² attemperatures of 130° C., 150° C., 170° C. and 190° C., respectively,using a 10 mm-wide seal bar, and then left to stand until cooled to theroom temperature. Next, 15 mm-wide specimens were cut out from the handsheets heat-sealed at said respective temperatures, and the heat-sealedarea of each specimen was peeled at the crosshead speed of 100 mm/min.,and thus was measured its peel strength (g/15 mm). The measured valuewas reported as the heat-seal strength.

[0112] (5) Hot Tack

[0113] Two sheets of double-layered hand made sheet were stacked in suchmanner that their heat-seal layers will touch face-to-face each otherand were heat-sealed for 0.5 sec under a pressure of 1 kg/cm² attemperatures of 130° C., 150° C., 170° C. and 190° C., respectively, andimmediately afterwards peel distance was measured by peeling theheat-sealed area under a load of 45 g. The measured peel distance wasrecorded as the basis on which to evaluate hot tack of the specimen.

EXAMPLE 1

[0114] Into an 80 liter autoclave equipped with a baffle plate and anagitator were charged 20 liters of n-hexane (23° C.), 20 liters of water(23° C.), 700 g of ethylene-methacrylic acid copolymer (resin 1), 300 gof high density polyethylene (resin 2) and 20 g of polyvinyl alcohol(PVA) (saponification degree 99%; viscosity of 4% aqueous solution (20°C.) 4.6 to 6.0 cps; manufactured by Japan Synthetic Chemical IndustryCo.; trade name: Gosenol NL-05). The liquid mixture was heated to atemperature of 145° C. with agitation using the agitator operated at 900rounds permin. Agitation was continued for 30 min., while the mixedliquid temperature was maintained at a temperature of 145° C. to finallyobtain a suspension.

[0115] Next, the suspension was flashed into a drum maintained in anitrogen atmosphere at a pressure of 400 mm Hg via a pipe connected to anozzle of a 3 mm diameter and 20 mm length provided on the autoclave,and thus was obtained a fiber-like material. The fiber-like material wasthen converted into an aqueous slurry of a 10 g/liter concentration in areceptacle, and the slurry was beaten/refined in a 12 inch-diameterdisk-type refiner, and thus was obtained a pulp-like material.

[0116] Properties of the synthetic pulp thus obtained were an averagefiber length (CFL) of 1.0 mm and a Canadian Standard Freeness (CSF) of670 cc.

[0117] As the raw material for the substrate layer, abaca fiber was madeavailable, and as the raw materials for the heat-seal layer, a mixturecomprised of 70% by weight of said synthetic pulp and 30% by weight ofpolypropylene fiber (fineness: 2 deniers; fiber length: 5 mm; meltingpoint: 165° C.) were made available. Next, the substrate layer having agrammage of 14 g/m² and the heat-seal layer having a grammage of 4 g/m²were formed into wet webs using a manually operated laboratory-scalesquare paper making machine. The two sheets in the form of wet web werestacked and a double layered paper was produced. Thereupon, the doublelayered paper was dried for 5 min. at a temperature of 50° C. using ahot air drier, and further underwent heat treatment for 1 min. at atemperature of 190° C.

[0118] The obtained double layered paper exhibited firm bond between thesubstrate layer and the heat-seal layer. This double layered paper wasreferred to tests for measuring its air permeability, heat-seal strengthand hot tack. The test results are shown in Table 1.

Example 2

[0119] A double layered paper was made available in the same manner asin Example 1 except that a polypropylene core & polyethylene sheathconjugate fiber (weight fineness: 2 deniers; fiber length: 5 mm)comprised of polyethylene (PE) (melting point: 130° C.) for the sheathcomponent and polypropylene (PP) (melting point: 165° C.) for the corewas used in place of the polypropylene fiber which was used in Example1.

[0120] The obtained double layered paper exhibited firm bond between thesubstrate layer and the heat-seal layer. This double layered paper wasreferred to tests for measuring its air permeability, heat-seal strengthand hot tack. The test results are shown alongside in Table 1.

Comparative Example 1

[0121] A double layered paper was prepared in the same manner as inExample 1 except that polypropylene fiber alone was used as the fiber toconstitute the heat-seal layer in place of the mixed fibers of syntheticpulp and polypropylene fiber which were used in Example 1.

[0122] The obtained double layered paper was referred to tests formeasuring its air permeability, heat-seal strength and hot tack. Thetest results are shown alongside in Table 1.

Comparative Example 2

[0123] A double layered paper was prepared in the same manner as inExample 1 except that the polypropylene core & polyethylene sheathconjugate fiber alone was used as the fiber to constitute the heat-seallayer in place of the mixed fibers comprised of synthetic pulp and thepolypropylene core & polyethylene sheath conjugate fiber which was usedin Example 2.

[0124] The obtained double layered paper was referred to tests formeasuring its air permeability, heat-seal strength and hot tack. Thetest results are shown alongside in Table 1. TABLE 1 Compara. Compara.Ex. 1 Ex. 2 Ex. 1 Ex. 2 Composition of Heat-Seal Layer wt. % Syntheticpulp 70 70 — — Polypropylene fiber 30 — 100 — Polypropylene core & — 30— 100 polyethylene sheath conjugate fiber Frazier 135 129 140 135 airpermeability (cm³/cm²/s) Heat-seal strength (g/15 mm) Sealing 130 132171 0 0 temp. (° C.) 150 188 202 36 182 170 230 225 210 188 190 224 223218 180 Hot tack (mm) Sealing 130 65 60 200 200 temp. (° C.) 150 55 55200 45 170 30 35 30 75 190 35 45 15 150

[0125] As can be clearly seen by comparison of the measured propertiesin Examples 1 and 2 and the measured properties in Comparative Examples1 and 2, the obtained double layered paper exhibited good airpermeability and at the same time the measured heat-seal strength was ashigh as 130 to 190° C. Besides, the peel distance which is the indicatorof hot tack was small, thus was found that it possesses good hot tack.

Industrial Applicability

[0126] The heat-seal paper of the present invention imparts a highinterlayer bond strength between the heat-seal layer and the substratelayer, as well as strong bond among fibers, for the reason that theheat-seal layer comprises the polyolefin synthetic pulp having thebranched configuration and preferably synthetic fiber, and also retainsgood air permeability. Since the heat-seal paper of the presentinvention possesses such heat-seal layer, it exhibits the stable andhigh degree of heat-sealability and hot tack over the wide temperaturerange extending from low temperatures to high temperatures, inparticular good heat-sealability and hot tack at low temperatures.

[0127] This heat-seal paper having good air permeability may be utilizedextensively and suitably as raw material paper for manufacturing variousbags such as filter bags. This heat-seal paper may be used for tea bags,bags to package sterilized apparatus and tools with, desiccators bags,etc. In cases where this heat-seal paper is used on high-speed automaticbag-making machine, stable operation and high productivity may besecured on account of its own ability to be heat-sealable over the widetemperature range, which ability being attributed to freedom frompossible adhesion to the hot plate in the molten state and good hottack.

[0128] Furthermore, the heat-seal paper of the present inventionpossesses superior physical properties, such as air permeability,heat-sealability, and hot tack, compared with the heat-seal paperconventionally in use as the raw material paper for the tea bagmanufacture. Moreover, it is principally comprised of polyolefin fiber,and hence there is few fear of generation of harmful substances likedioxines when incinerated as wastes. Hence, it contributes to protectionof the environments.

What we claim is:
 1. A heat-seal paper having air permeability, whichcomprises a laminated body integrating a heat-seal layer (A) principallycomprised of a synthetic fiber having a branched configuration and asubstrate layer (B) principally comprised of a natural fiber.
 2. Aheat-seal paper having air permeability according to claim 1, whereinsaid synthetic fiber having the branched configuration is a polyolefinsynthetic pulp.
 3. A heat-seal paper having air permeability accordingto claim 2, wherein said polyolefin synthetic pulp has an average fiberlength of 0.1 to 10 mm and a freeness of 700 cc or less.
 4. A heat-sealpaper having air permeability according to claim 2 or 3, wherein saidpolyolefin synthetic pulp is constructed of a resin compositioncomprised of 50 to 100% by weight of an ethylene-α,β-unsaturatedcarboxylic acid copolymer containing 1 to 20% by weight of anα,β-unsaturated carboxylic acid and 0 to 50% by weight of a polyethyleneresin.
 5. A heat-seal paper having air permeability according to eitherone of claims 1 to 4, wherein said heat-seal layer (A) is comprised of50 to 100% by weight of the polyolefin synthetic pulp and 0 to 50% byweight of at least one type of fiber selected from the group consistingof polyethylene fiber, polypropylene fiber, polyester fiber andconjugate fiber thereof.
 6. A heat-seal paper having air permeabilityaccording to either one of claims 1 to 4, wherein said heat-seal layer(A) is comprised of 50 to 90% by weight of the polyolefin synthetic pulpand 10 to 50% by weight of at least one type selected from the groupconsisting of polyethylene fiber, polypropylene fiber, polyester fiberand conjugate fiber thereof.
 7. A heat-seal paper having airpermeability, which is a laminated body integrating a heat-seal layer(A) and a substrate layer (B) principally containing a natural fiber,wherein said heat-seal layer (A) is comprised of 1 to 99% by weight of apolyolefin synthetic pulp having a branched configuration and an averagefiber length of 0.1 to 10 mm and 1 to 99% by weight of a synthetic fiberhaving a fineness of 0.1 to 10 deniers and an average fiber length of0.1 to 30 mm, and said synthetic pulp is constructed of a resincomposition comprised of 50 to 99% by weight of anethylene-α,β-unsaturated carboxylic acid copolymer containing 1 to 20%by weight of an α,β-unsaturated carboxylic acid and 1 to 50% by weightof a polyethylene resin.
 8. A heat-seal paper having air permeabilityaccording to claim 7, wherein said heat-seal layer (A) is comprised of50 to 99% by weight of the polyolefin synthetic pulp and 1 to 50% byweight of the synthetic fiber.
 9. A heat-seal paper having airpermeability according to claim 7, wherein said heat-seal layer (A) iscomprised of 50 to 90% by weight of the polyolefin synthetic pulp and 10to 50% by weight of the synthetic fiber.
 10. A heat-seal paper havingair permeability according to either one of claims 7 to 9, wherein saidsynthetic fiber is at least one type of fiber selected from the groupconsisting of polyethylene fiber, polypropylene fiber, polyester fiberand conjugate fiber thereof.
 11. A heat-seal paper having airpermeability according to either one of claims 7 to 10, wherein saidethylene-α,β-unsaturated carboxylic acid copolymer is anethylene-acrylic acid copolymer or an ethylene-methacrylic acidcopolymer.
 12. A heat-seal paper having air permeability according toeither one of claims 7 to 11, wherein said polyethylene resin has adensity of 0.941 to 0.970 (g/cm³) and a melt flow rate of 0.1 to 100(g/10 min.).
 13. A heat-seal paper having air permeability according toeither one of claims 7 to 12, wherein said polyolefin synthetic pulp hasa freeness of 700 cc or less.
 14. A heat-seal paper having airpermeability according to either one of claims 7 to 13, wherein saidheat-seal layer (A) has a thickness corresponding to a grammage of 1 to10 (g/m²).
 15. A heat-seal paper having air permeability according toeither one of claims 7 to 14, wherein said substrate layer (B) has athickness corresponding to a grammage of 10 to 30 (g/m²)
 16. A heat-sealpaper having air permeability according to either one of claims 7 to 15,wherein said substrate layer (B) is comprised of an abaca pulp.
 17. Aheat-seal paper having air permeability according to either one ofclaims 7 to 16, wherein said laminated body is comprised of 15 to 30% byweight of the heat-seal layer (A) and 70 to 85% by weight of thesubstrate layer (B).
 18. A heat-seal paper having air permeabilityaccording to either one of claims 7 to 17, wherein said laminated bodyhas a thickness corresponding to a grammage of 11 to 40 (g/m²).
 19. Aheat-seal paper having air permeability according to either one ofclaims 7 to 18, wherein said laminated body has a Frazier airpermeability of 100 to 300 (cm³/cm²/s)
 20. A heat-seal paper having airpermeability according to either one of claims 7 to 19, wherein saidlaminated body has a heat-seal strength at a temperature of 130° C. of100 to 300 (g/15 mm).
 21. A heat-seal paper having air permeabilityaccording to either one of claims 7 to 20, wherein said laminated bodyhas a hot tack as represented by a peel distance at a temperature of130° C. is 1 to 100 mm.
 22. A heat-seal paper having air permeabilityaccording to either one of claims 7 to 21, wherein said laminated bodyis a filter bag paper.
 23. A heat-seal paper having air permeabilityaccording to either one of claims 7 to 21, wherein said laminated bodyis a tea bag paper.
 24. A heat-seal paper having air permeabilityaccording to either one of claims 7 to 23, wherein said heat-seal layer(A) and said substrate layer (B) constituting the laminated body do notsubstantially contain any halogen atom.